1. Field of the Invention
The present invention relates to an optical fiber cord and an optical cord ribbon.
In accordance with the recent progress of optical telecommunication technology, there is an increase in the number of requests for employing optical fiber cables in the subscribers' system including personal or regular subscribers, and there will be requirements of as many as 100,000 fibers in a central office in the near future.
In order to avoid the optical fibers getting damaged during the cable distributing operation in central offices, an optical fiber cord of a specific structure, such as the one being formed by applying reinforcing members of high-resiliency along the longitudinal direction of a coated fiber, and by coating synthetic resin therearound, is needed.
2. Description of the Related Art
FIG. 20 shows an explanatory view showing a sectional surface of a conventional mono-fiber type optical fiber cord. FIG. 21 shows a multi-fiber type optical fiber cord. The mono-fiber type optical fiber cord 3 in FIG. 20 (hereinafter referred to simply as "mono-fiber cord") shows a structure that an optical fiber 11 and a coat 32 applied thereto (hereinafter referred to as an "optical-fiber coat" or just "fiber coat") configures a coated optical fiber 33, and a reinforcing member 34 and an outer sheath 35 are applied to the outermost side thereof. The outside diameter of a standard coated fiber is normally 0.25 to 0.9 mm, and that of the optical fiber cord 3 containing the coated fiber is at least 1.7 mm or so. The reinforcing member 34 (hereinafter may be referred to just as "strength member") is, in most occasions, made of aramid yarn. In this way, the optical fiber cord 3 is made much larger in size than the coated optical fiber 33 itself. Accordingly, since cables formed by assembling a plurality of mono-fiber cords are conventionally used for an office-site cable distribution, there has been a problem that if the number of cords to be assembled is increased, the total size thereof is made extremely large, and thus a lot of space for cable distribution is needed.
The multi-fiber type optical fiber cord 4 (hereinafter referred to simply as a "multi-fiber cord") shown in FIG. 21 is constructed such that four coated optical fibers 33 each formed by an optical fiber 11 and an optical fiber coat 32 are laterally aligned (in the case of the figure), a bundling coat 41 is applied integrally to the thus aligned coated fibers 33 to form an optical fiber ribbon, the external face of the thus formed fiber ribbon is supported by aramid yarn in the longitudinal direction thereof as a strength member 42, and further an outer sheath 43 made of synthetic resin is applied therearound. The multi-fiber cord 4 formed in the above manner normally adopts four-fiber or eight-fiber optical fiber ribbon therein. Since the sectional area per fiber of the multi-fiber cord is much smaller than that of the conventional mono-fiber cord, it will be advantageous in that a high-density accommodation of the optical fibers is enabled if it is used as an office-site optical fiber cable. However, since the connections for all the fibers are to be changed by use of connectors at the main distribution board within the office site, there causes such a problem that the multi-fiber cord must be converted to individual mono-fiber cord. Note that the wording "connection" or "connecting" includes the meaning of "splice" or "splicing" throughout the present specification.
Conventionally, two methods have been adopted to perform a mono-cord to multiple-cord conversion or the other way round (hereinafter referred to just as a mono-multi or multi-mono cord conversion). One of the methods is to form a connection point, which, though, will raise a total cost as it requires for connecting operations and connecting members. In addition, there are also such problems as necessity of accommodation of the connecting point, an increase of optical loss and so on. The other method is to open up the leading end of the multi-fiber cord to divide it to a plurality of individual coated fibers, and thereafter cover each of the coated fibers by tubes to form a plurality of optical fiber cords. Although this method does not cause such a problem as an increase of optical loss due to the fact that there is no need for a connecting point therein, the dividing operation requires for a high-leveled manual skill, and in addition, in the case that even only one fiber is damaged, the entire multi-fiber cord including the damaged portion must be cut to a required length and execute the same manual operation all over again, so that when an optical cord of a predetermined length is designated, the total cost will be extremely raised.
In order to solve the aforementioned problems such as the increased size of mono-fiber cord, the necessity of mono-multi or multi-mono cord conversion and so on, there has been proposed a new optical fiber cord constructed as a trial structure, which is disclosed in a paper "a small-diameter cord with increased flexural rigidity" by Hajime Takemoto, and Masao Tachikura (Discourse number B10-76, page 585 of volume 2 of Proceedings of the 1998 IEICE General conference, published by the Institute of Electronics, Information and Communication Engineerings on Mar. 6.sup.th, 1997).
One of the structures disclosed in the paper is shown in FIG. 22, which has an outside diameter of 0.25 mm, and the diameter of the coated optical fiber itself is 0.15 mm. Although this fiber cord has completed the purpose of minimizing the diameter thereof, as a plurality of steel wires (12 wires in the figure) each having a diameter of 0.045 mm are provided around the coated optical fiber, the removing operation of the coat for the optical fiber is made more difficult. Further, as the outside diameter of the coated fiber is made smaller than 0.25 mm, the fiber coat is also made thin and thus it gets readily damaged.